A multi-scale capillary-core-reservoir approach to determining cluster spacing for volume fracturing: A case study of Chang 7 shale oil of Triassic Yanchang Formation, Ordos Basin, China

Abstract

This paper proposes an approach to determining the optimal cluster spacing for volume fracturing in shale oil reservoirs based on three scales, i.e. microscopic capillary displacement, large-scale core imbibition, and macroscopic reservoir nuclear magnetic resonance (NMR) logging. Through flow experiments using capillary with different diameters and lengths, and large-scale core counter-current and dynamic imbibition tests, and combing with the NMR logging data of single wells, a graded optimization criterion for cluster spacing is established. The proposed approach was tested in the shale oil reservoir in the seventh member of the Triassic Yanchang Formation (Change 7 Member), the Ordos Basin. The following findings are obtained. First, in the Chang 7 reservoir, oil in pores smaller than 8 µm requires a threshold pressure, and for 2–8 µm pores, the movable drainage distance ranges from 0.7 m to 4.6 m under a pressure difference of 27 mPa. Second, the large-scale core imbibition tests show a counter-current imbibition distance of only 10 cm, but a dynamic imbibition distance up to 30 cm. Third, in-situ NMR logging results verified that the post-fracturing matrix drainage radius around fractures is 0–4 m, which is consistent with those of capillary flow experiments and large-scale core imbibition tests. The main pore-size range (2–8 µm) of the Chang 7 reservoir corresponds to a permeability interval of (0.1–0.4)×10^-3 µm^2. Accordingly, a graded optimization criterion for cluster spacing is proposed as follows: for reservoirs with permeability less than 0.20×10^-3 µm^2, the cluster spacing should be reduced to smaller than 4.2 m; for reservoirs with permeability of (0.2–0.4)×10^-3 µm^2, the cluster spacing should be designed as 4.2–9.2 m. Field application on a pilot platform, where the cluster spacing was reduced to 4.0–6.0 m, yielded an increased initial oil production by approximately 36.6% over a 100-m horizontal reservoir section as compared with untested similar platforms.